Carrier current system



May 30, 1933. M. L. NELSON Er AL 1,911,712

CARRIER CURRENT SYSTEM Filed March so, 1928 5 Sheets-Sheet 1 MOTOR START i CONTAC'I'ZJR LINE JLOA DING Il-o- Imren DPS F T-mm LNalsnn and HE EHerfi y 1933. M. L. NELSON ET AL 1,911,712

CARRIER CURRENT SYSTEM Filed March 30, 1928 5 Sheets-Sheet 2 Imam MERE LNalsun and HE E.HE1"iiEj y 30, 1933- M. L. NELSON ET AL 1,

CARRIER CURRENT SYSTEM Filed March so. 1928 5 Sheets-Shet 3 ---Ir+ nr s;--- P'Tar' 1111 L. Na lean and Harry EHsrshe I H11 May 30, 1933. M. L. NELSON ET AL ,9

CARRIER CURRENT SYSTEM Filed March so, 1928 5 Sheets-Sheet 4 May 30, 1933.

M. L. NELSON El AL CARRIER CURRENT SYSTEM Filed March 30. 1928 5 Sheets-Sheet 5 and P533 EZHET'E'LIE each being Patented May 39, I933 lJNETli.

MARTIN L. NELSON, 0F PARK RIDGE, an]; HARRY n. rrnnsriEY, or 0A1; PARK, ILLINOIS} ASSIGIIORS, BY IVJ'LESNE ASSlEG-NIYIENTS, TO ASSGCIATED ELECTBIG LABORATORIES,

INC 3F CHECELGO, ILLINOZS, A COItPORATIGll OF DELAWARE CARRIER CUR-RENT SYS TEIVI Application filed March 30, 1923. Serial No. 265,970.

Our invention relates to improvements in carrier current systems.

Such systems utilize power transmission lines over which the carrier current is transmitted from one power station to another.

By means of carrier current equipment comprising radio transmitters and receivers, the carrier current forms a path for signal= ling and conversational purposes.

Each station in addition to the carrier current equipment is provided with automatic switches of the type, illustrated and described in the publication Telephony Tncluding Automatic Switching byv Smith, pages 212 and 213. published January, 1924.

Each transmitter is provided with an automatic switch and a group of relays for controlling outgoing calls from one station to another. This same switch also establishes connections between telephones located in the same exchange.

In some instances. two individual power companies have their lines tied together and the carrier current telephone equipments opcrating on different frequencies of current.

In such cases the radio transmitter is arranged for transmitting signalling and speech currents on the frequency of each system. Two radio receivers are provided, tuned to respond to one of the frequencies.

Each receiver is arovided with an automatic switch which responds to incoming calls on a particular frequency for extending a call to the desired telephone. Each telephone is of the two wire automatic type, being provided with a dial for transmitting impulses by interrupting the line circuit to control the automatic switches.

The impulses transmitted by the dial of a telephone operate the switch for cont-rolling the radio transmitter at the originating office. i r

A group of preliminary impulsesare first transmitted and the switch in response thereto prepares the carrier equipment for operation. The impulses following the preliminary group or set are received at a distant station over a path formed by the carrier current.

These impulses depending upon the frequency utilized control an automatic switch throughthe radio receiver tuned to that frequency. The operation of this switch selects the called telephone, applies ringing current andfurnishes current for speech transmission. i

Our invention is illustrated in the accompanying drawings wherein:

Fig. 1 shows a dispatchers station provided with an automatic telephone andan automatic switch provided with a set of con trolling relays. I

F ig.'2 shows two automatic switches in the same'office for receiving incoming calls from other stations over power lines having different frequencies.

Fig. 3 shows a radio transmitter circuit for controlling the carrier current.

Fig. 4c shows a radio receiver for incoming calls responsive to a certain frequency.

Fig. 5 shows a similar radio receiver for receiving incoming calls responsive to a different frequency.

The automatic switch T1 shown in Fig. l is provided with four wipers and two banks. Each bank consists of ten levels, each level comprising ten pairs of contacts. The switch shaft carries two line wipers designed to engage contacts of the line in the line bank and also carries a pair of private wipers arranged to engage the contacts in the private bank. A multiple of each line is carried to the contact in the line bank which corresponds to the number of that particular line. Corresponding contacts in the other banks are wired to the various cut-ofi' relays associated with the lines.

Certain contacts in the switch banks are left blank or disconnected from any circuit in order that no effect will be produced when calling the first two digits; The switchwipers, excepting when calling locally or for a telephone in the same oflice, will engage these blank contacts without effect. In other words, the switch in selecting such blank con tacts prepares certain circuits in conjunction with the group of control relays to prepare the carrier equipmentfor operation. 7

The line bank contacts and cut-offrelay contacts are multipled to corresponding contijacts in the banks of switches T2 and T3 of The relays of the group at the top of Fig. 1 are associated with the three switches T1, T2, and T3, and serve to control the carrier current equipment for outgoing and incoming calls. The switch T1 is used for outgoing calls and also for local calls.

The switches T2 and T3 are for receiving incoming calls from other stations. One switch is utilized for a current of a-certain frequency and the other is utilized for current of a different frequency. These two frequencies are termed A and B frequencies, respectively. The dispatchers telephone 86, Fig. 1, is of the standard two-wire automatic type, and the two line conductors at the telephone eX- tend to a line relay L through the contacts of the cut-off relay M. These two relays are mechanically interlocked and serve as line switches to connect the telephone line conductors to the carrier current equipment.

The co-operation of these two relays is described in Telephony Including Automatic Switching by A. B. Smith, published 1924, pages 229-232, inclusive.

A second telephone and line equipment 77 is shown, which is the same as the equipment of the dispatchers line 86 except that the normally closed contact of relay M1 which furnishes ground for operating relay L1 is connected to ground through the back contact of relay A of the relay group instead of being connected directly to ground as in the case ofrelay M. i All other lines have the same equipment and are wired the same as line 77. Each line, of which there may be any number, is equipped with standard lineloading apparatus.

The radio transmitter, Fig. 3, and the radio receivers of Figs. 4 and 5 in themselves form no part of our invention, but are used for purposes of illustration, as any similar transmitter or receivers may be used.

In Fig.3, the two amplifiers 63 and 68 and the modulator 70 co-operate with the master oscillator 78 and power amplifier 79 for regulating the currents carried over the power line conductors and 41. These conductors terminate in another station or stations which are provided with the same kind of equipment illustrated in Figs. 1 to 5, inclusive.

' The alternator 87 whenplaced in operation, connects current to the carrier under control of the switch T1, Fig. 1, to operate an automatic switch at the distant station corresponding to T2 or T3 which control the complete the circuit of either variometer 84 or 85. The former is used for A frequency current, while the latter is used for B frequency current.

The receiver, Fig. 4, is tuned to respond to current of one frequency, termed the A frequency, while the receiver, Fig. 5, is tuned to respond to current of a different frequency, termed the B frequency.

The receiver, Fig. 4, is provided with a detector 104 and three amplifiers 105, 106, and 107. The amplifier 107 is used for operating relay R associated with switch T3, Fig. 2. The operation of this relay in response to impulses transmitted over the carrier current from the distant station causes the switch T3 to operate and select the proper telephone, after which ringing impulses are applied to the selected telephone. The tube 109 has its filament controlled by relay G, Fig. 1. Thistube acts as a protector, as its plate is connected'to a circuit extending to the grid of the tube 104 to prevent excess current entering the receiver.

The tube 110 is utilized for controlling the operation of the call-timing relay B, Fig. 1. The tube itself is controlled initially by relay C, termed the time-control relay, Fig. 1.

The receiver of Fig. 5 is similar to that of Fig. 4, being provided with the detector 142 and the three amplifiers 143, 144, and 145. The latter amplifier controls the relay associated with the switch T2, which is operated over the B frequency current from a distant station.

The detailed operation of the circuits will be pointed out in the description of operation.

We will first describe the operation of the system when the dispatcher or operator whose telephone is No. 86 desires to converse with a telephone No. 55 which has a code of one short and one long ring.

- Assuming contacts 44 in the switch banks are blank, then the calling party will dial 4%552-0. The digits 4-4 will cause the switch to select contacts 44, digits 55 will be transmitted over the carrier current to operate a. switch at another oflice to select telephone No.55, while digits 20 will cause the code ringing relay to apply ringing current for one short and one long ring, thus notifying N0. 55 he is wanted.

As is customary in carrier systems of this character, suitable coupling devices such as condensers or coils are interposed in conductors 40 and 41 so as to inductively separate them from the main conductors carrying the high voltage power current. 1

The switch and apparatus at the distant station on an outgoing call are identical with and operated as described in connection with the operation of switch T3'for an incoming call using A frequency. This operation is described in detail under the heading, in

coming call using A frequency, and will not be described in the present instance.

The sheets of drawings are arranged with Figs. and 2 end to end and placed below Figs. 3, 4. and 5 which are likewise placed end to end.

Outgoing call using A frequency When the calling subscriber at 86 removes his receiver, relay L will operate and close the circuit of relay M. As relay L is slow in restoring, both relays are operated during the same time interval, thus allowing relay M to become completely operated, disconnect relay L, and connect the line through to the line relay A1 of switch T1. This latter relay will now supply talking current to the calling subscriber, and becoming energized. will in turn operate relay B1 which will con nect ground to the winding of M through its locking contacts to maintain it in its energized position. The operation of relay M also connects ground to relay A and it, upon operating, connects 125-volt current to the tone, alternator 87, Fig. 3, to place it in operation. Relay A also connects ground to the winding of relay H through the contacts of relay D, and the relay H, upon operating. closes the circuit of the motor-start relay K and closes the filament circuit of tube 109, Fig. 4, through the contacts of re lay G by way of conductors and 15.

The operation of relay A prevents additional calls being made at this time, as it removes positive battery from the contacts of all idle relays such as M1 but does not re move ground from the contact of relay M. The contact of relay M is connected directly to ground in order that the dispatcher at station 86 can be connected to the connector T1 at any time regardless of whether the connector is busy or not. However, all other stations can be connected to the connector only if it is idle.

Relay K, which is operated by the auxiliary motor-start relay H, is provided with a copper slug at its armature end, and is of high impedance. The armature of relay K will not be operated for an appreciable time following the closure of the circuit of relay H by relay A. As relay A connects positive 48-volt battery to the movable contact of the frequency change position relay provided with magnets N and 0', it naturally follows that this current will be connected to the contact for an appreciable time.

Belay K also completes the circuit of the motor-start relay. This latter relay is not shown, but is of suliicient size and construction to supply current to operate a motor generator which in turn supplies current to the various tubes and other equipment which does not require current supply until the equipment is actually in use or ready to be used. This provislon 1s made'for the purpose of saving current and prolonging the life of the equipment.

Relay A1 also connects ground to the winding of the time control relay C, Fig. 1, which upon operating removes the negative bias battery from the grid of tube 110, Fig. 4. Relay B becomes energized from the plate circuit of this tube and disconnects the tone alternator signalling current and closes the microphonecircuit through the windings 64 and 67 of the transformers, Fig. 3.

From an outgoing call on A frequency current, relay S, Fig. 3, is not operated so the A frequency variometer 8% will cooperate with the oscillator 7 8, Fig. 3.

In order to prepare the outgoing circuit to extend a call to the substation, the number 4 1 or any other idle number or contact in the banks of switch T1 is dialled.

When the calling party dials the first digit, the line relay A1 of the connector T1 will transmit four impulses of current to the vertical magnet F1 in series with low resistance winding of relay C1, causing the wipers to be raised to a position opposite the fourth level of bank contacts.

At the first vertical step of the shaft, the off-normal springs close the circuit of the olfnormal relay F of the relay group.

lVhen the'calling subscriber dials the second digit, the line relay A1 of the connector operates'the rotary magnet H1, causing the shaft to rotate and Wipers engage bank contacts 44. When the code ringing relay D1 of the switch Tl deenergizes following the last rotary impulse transmitted, a circuit is closed from positive t8-volt battery at the ofi-normal springs, upper winding of the switching relay E1, break springs of the code ringing relay D1, private wiper P2 and the contact upon which it is standing, through a non-inductive resistance of the relay group to negative 4e8-volt battery. The closing of the circuit through the upper winding will cause the armature of the switching relay E1 of the switch T1 to close the springs X, whereupon a circuit is closed from positive l8-volt battery at the ofi-normal springs, the make springs of the switching relay E1, and finally throughthe lower winding of the same relay to negative 48- volt battery.

The closing of the circuit of the lower winding will cause the switching relay E1 to become fully operated, whereupon the circuit of the rotary magnet will be opened, thus preventing further rotation of the shaft of the switch 'll. A further result of the energization of the switching relay E1 causes the closure of a circuit extending from one terminal of the tone alternator through the transformer winding 65, break contacts of the call timing relay B of the relay group, make springs of the line relay A1 of the connector, make springs of the switching relay E1, to the other terminal of the tone alternator. A further circuit is closed from positive AS-volt battery at the line relay A1, make springs of the switching relay E1 through the time control relay C of the relay group to negative 48-volt battery.

The calling party will now dial diglt 5, whereupon the line relay A1 will respond and transmit five impulses of current through the code ringing relay D1. The latter relay will energize and remain operated while the impulses are passing through it, but no effect occurs at this time, as the line wipers are standing upon open or idle contacts 44. As the line relay A1 falls back following each impulse of the series, the circuit to the time control relay C of the relay group is momentarily opened, whereupon this relay deenergizes and re-establishes a bias condition on the grid of the tube 110. A further result of the operation of relay A1 opens the tone alternator circuit five times, thus transmitting five impulses over the carrier circuit. The digits 1% were not sent out over this circuit, because of the open circuit at the contacts of the switching relay E1.

The alternator circuit passes through a winding 65 of the transformer Z, thus applying induced current to the grid of the amplifier 68. Five impulses of current are superimposed on the carrier current and are received by the automatic switch at the distant station which is associated with the power line formed by conductors 11 and 10.

The callin party now dials the digits 520, thus completing the connection and giving the code ring. The operation of the relays previously described will operate in the same manner in response to the transmittal of these digits.

Approximately three seconds following the dialling of the last digit, the bias of the grid of tube 110 will gradually fade away and allow the call timing relay B of the relay group to energize, whereupon the circuit of the tone alternator is opened. At the same time, the microphone circuit is closed, allowing conversation to take place from the calling to the called telephone.

The outgoingor speech transmission circuit from station or telephone 86 extends to conductors 1 and 2 through the contacts of relay M. hese conductors extend to the hybrid coil T, 4, and pass through the lower winding in series with the two condensers as shown. The circuit then extends by way of conductors 33 and 34 to the trans former winding 61, Fig. 3. The current then passes through the secondary winding 62 of the transformer to the grid of the tube 63,

primary winding of the transformer associated wlth the plate of this tube, then to the secondary winding 64: which is closed in series with the primary winding 67 of transformer Z through the contacts of relay B and the condenser shown in Fig. 1. At this time the microphone circuit extending through the winding 67 of the transformer is closed, whilewinding 65 through which the signals were transmitted is now opened at the contacts of relay B, Fig. 1.

The talking circuit induced through the secondary winding 66 of the transformer extends to the grid of tube 68, thence through transformer 69 to the grid of the modulator tube 7 0'. i

As relay S, Fig. 3, is in its normal position, the variometer 8% acts in conjunction with the master oscillator 78, which has its current amplified by the tube 7 9. This amplified current from the oscillator is modulated by the tube 70, so that properly modulated talking current passes over the carrier by Way of conductors 40 and 4:1 to the distant station.

From the foregoing it will be seen that the carrier equipment controlled from the station 86, Fig. 1, places the transmitting equipment, Fig. 3, in operation to form a conversational path from the calling party to the carrier current equipment. l/Vhen the calling party restores his receiver the circuit of the line relay A1 is opened and it, on deenergizing, opens the circuit of the release relay B1 of the switch T1 and the time control relay C of the relay group. The deenergization of relay B1 will open the circuit of the relay M and the lookout relay A of the relay group.

The relay A, restoring, will open the circuit of the auxiliary start relay H, which in turn will open the circuit of the motor start relay K. The off-normal relay F will also deenergize when the off-normal springs assume their normal position, as will the switching relay E1 of the switch T1.

Local calls In describing the operation of the system when calling for a telephone in the same sta tion, we will assumethat station No. 77 desires to'call station 86, followedby the code ring 20, which represents one short and one long ring.

The operation of the apparatus following the removal of the receiver of the calling subscriber and the dialling of the first and second digits is the same as previously described, the only difference being that the wipers have been brought into engagement with bank contacts 86.

When the code ringing relay D1 of the switch T1 deenergizes following the last rotary impulse, a circuit is closed from positive 18-volt battery and off-normal springs through the upper winding of the switching relay E1, break contacts of the code ringing relay D1, private wiper P2 and the contact upon which it is standing, through the relay M associated with the line 86, to negative 48 -v0lt battery.

The switching relay El will completely energize, as before described, and will shunt out or short-circuit the break contact springs of the code ringing relay D1, thus maintaining the circuit to relay M closed when the code ringing relay D1 responds to further series of impulses.

The relay M will energize over the previously traced circuit and will clear the called line.

A further result of the operation of the switching relay E1 closes the circuit of the ringing motor generator. A pair of make contacts on the switching relay E1 closes the line wipers through a pair of break contacts on the code ringing relay D1 to the negative and positive terminals of the impedance coil E of the relay group. This coil is then in position to feed talking current to the called station.

A further result of the operation of relay El closes the circuit from positive 48-volt battery to the private wiper P1 and the con- ,tact upon which it is standing, through the winding of the relay D of the relay group to negative battery. The relay D, upon energizing, opens the circuit of the auxiliary motor start relay H, whereupon it and relay K deenergize, thus stopping the motor and preventing the possibility of the talking circuit-s singing or howling, which would be brought about by oscillations set up in the circuit.

A further result of the operation of relay D connects the negative and positive terminals of line relay A1 to the condensers and to the negative and positive sides, respectively, of the impedance coil E.

The calling subscriber will now dial the digits 20 in order to produce a code ring. The line relay A1 will respond to the dial-' ling of the digit 2 and will transmit two impulses of current to the ringing relay D1, thus maintaining this relay energized for a short period of time. The code ringing relay D1 upon energizing, will cause ringing current to be connected to the called line through a pair of make contacts of the ringing relay D1, a pair of make contacts of the switching relay E1, a pair of line wipers and the bank contacts upon which they are stand- 7 ing, thus extending current to the telephone of the called line.

The line relay A1 will respond to the dialling of the digit 0 and will transmit ten impulses of current through the code ringing relay D1, thus maintaining the latter relay energized for a comparatively long period of time. The operation of the ringing relay D1 will, as before, serve to connect ringing current to the called line.

When the called party answers, he will receive talking current from the impedance coil of the relay group, through a pair of break contacts on the code ringing relay D1 and over the circuit previously traced for the ringing current. The transmission will be effected through the two condensers which connect the positive and negative terminals of the line relay A1 of the switch T1 to the positive and negative terminals, respectively, of the impedance coil E of the relay group.

lVhen the calling party restores his receiver, the circuit of the line relay A1 of the connector will be opened, whereupon the Incoming call using A frequency current The subscriber at a distant station wishing to call telephone number 86, Fig. 1, W111 remove his receiver and first dial preliminary impulses as outlined in connection with an outgoing call from telephone 86, Fig. 1. Tiese preliminary impulses or digits will operate a switch at the distant station corresponding to switch T1, which co-operateswith a set of control relays, as shown in Fi g. 1. These preliminary impulses and the operation of the switch and relays prepares the carrier current equipment and completes the tone alternator circuit as described. v

Following the preliminary impulses, four digits-8620 are transmitted, the first two digits corresponding to the-number of telephone 86, while the remaining digits for code ringing produce one short and one long ring at this telephone.

The digits 86 are transmitted by way of the carrier current over conductors 40 and 41, and are received at the A frequency receiver, Fig. 4, over conductors 35 and 86, and

pass through the primary winding 101 of the transformer U, Fig; 4, in series with the pri mary winding 140 of the transformer W, Fig. 5. The tone alternator current while passing in series through the primary windings of these two coils has no effect upon the receiving equipment of Fig. 5, as it is not tuned for the A frequency current, but the current is received by the equipment, Fig. 4.

The current passes through the primary winding 101 to the secondary winding 102 and from there to the grid of the tube 104. From this tube the current passes from the plate circuit to the primary winding 111 to the secondary winding112 of transformer V, and is amplified by the three tubes 105, 106, and 107 The current after being finally amplified by the tube 107 causes the energization of relay R, Fig. 2, associated with the A ,frequency switch T3. The operation of relay R completes the circuit of the line relay A3. As eight interruptions of the amplified tone alternator current are received by relay B, it releases eight times and in turn causes the line relay A3 to release a corresponding number of times, thereby causing the switch T3 to step Vertically with its wipers opposite the eighth row of bank contacts. Following this group of impulses, the next series of six impulses are likewise received at the switch T3, causing it to rotate and bring its wipers in engagement with the sixth set of contacts in the eighth row. It is not necessary to describe in detail the operation of the switch T3, as it operates in the same manner as switch T1 Fig. 1.

The line wipers of switch T3 will engage contacts extending to line 86, Fig. 1, while the winding of the switching relay E3 will be placed in series with the winding of the cut-ofi relay M of line 86 through the private wiper and the contacts which it engages. Relay E3 connects ground from contacts of relay A3 to the winding of magnet N and causes the contacts to disconnect conductors .21 and 27 to prevent the operation of relays G and J. These relays are only operated in connection with a B frequency operation.

Relay M will not fully operate at this time, due to its mechanical structure. The opera- .tion of relay E3 connects 125-volt current to of ringing current.

Approximately three seconds after the. transmittal of the last digit, the relay at the distant station corresponding to relay B, Fig. 1, will restore, as previously described in connection with an outgoing call. The restoration of this relay will disconnect the alternator or signalling current and close the microphone circuit for conversational purposes, also previously described. After the disconnection of the alternator current, relay R of switch T3 will restore and open the circuit of relay A3. The restoration of this latter-relay will allow relay B3 to restore and complete the circuit of the release magnet of switch T3, causing it tooperate and restore the switch T3 to its normal position. The restoration of the switch T3 will open the circuit of relay M of line 86. The restoration of relay M will allow relay L to be reconnected tothe line and to be in position to be operated when the called party answers, and when this happens, relay L will operate and close the circuit of relay M. As relay L is slow in restoring, both relays are operated, thus allowing relay M to become completely operated, disconnect relay L, and connect the line through to the line relay A1 of switch T1. This latter relay will now supply talking current to the called subscriber, and becoming energized, will in turn cause relay B1 to operate and connect ground to relay M to maintain it energized. The operation of relay M also connects ground to relay A and it, upon operating, connects 125-volt cur rent to the tone alternator, Fig. 3, to place it in operation. Relay A also connects ground to the winding of relay H through the contacts of relay D, and relay H, operating, closes the circuit of the motor-start relay K and connects 6-volt battery to the filament of gized from the plate circuit of this tube and disconnects the tone alternator signalling current and closes the microphone circuit through the windings of transformers 64 and 65, Fig. 3.

At this stage, we have the switch at the distant station corresponding to switch T1, Fig. 1, in its operated position and have the tone alternatorcurrent disconnected and the microphone circuit closed.

At the incoming end, we have switch T3, Fig. 2, restored, and in Fig. 1 relays M, A1, B1, A, B, C, H, and K operated and the switch T1 in its normal position.

The transmission circuit of the calling subscriber at the distant station is over the carrier current by way of conductors 40 and 41, through the windings 101 and 140 of the transformers U and W, Figs. 4 and 5. The

current passingthrough winding 101 is induced in the secondary winding 10?. of the 1 transformer U, and this current is received at the grid of the detector tube 104 and passes from there to the output or plate circuit through winding 200 of the transformer V, Fig. 4.

At this time, winding 111 of the transformer V through which the signalling or alternator tone current passed is short-circuited through the contacts of relay H by. way of conductors 17 and 18. This current passing through winding 200 induces current in the winding 113 which is in series with winding 148 of the transformer Y, Fig. 5. At this time the primary winding 201 of the transformer Y, Fig. 5, is short-circuitcd through the normally closed contacts of relay J, Fig. 1, by way of conductors 25 and 26.

The current, therefore, passing through winding 200 of the transformer, Fig. 4, will have no efl'ect upon the transformer Y of Fig.

5, but the current will have effectupon the transformer winding of the coil T, Fig. 4:. As the current passes through the primary winding 121 of this transformer, it induces current in the windings which are connected to conductors 1 d 2 which extend to called line 86 through the contacts of relay M associated with that line.

Incoming call using B frequency The subscriber a distant station wishing to call telephone No. 86, Fig. 1, will remove his receiver and dial preliminary impulses as outlined in connection with extending a call from telephone No. 86, Fig. 1, to a distant station. These preliminary impulses or digits will operate a switch at the distant station corresponding to switch T1, Fig. 1, which co-operate with a set of control relays, as shown in Fig. 1. These preliminary im pulses and the operation of the switch and relays complete the tone alternator circuit as described.

Following the preliminary impulses, four digits S62O are transmitted, the first two di its of which correspond to the number of telephone No. 86, while the remaining d gits produce one short and one long ring at this telephone.

The digits 86 are transmitted by way of of the carrier current over conductors 10 and ll, and are received at the 3 frequency receiver, Fig. 5, through the winding 101 of the transforms U, Fig. 1, in s ries with the windmg 110 of the transformer Fig. 5. The tone alternator current while passing in series through the primary windings of these two coils has no effect upon the receiving equipment of Fig. as it is not tuned for this frequency, but such mpulses are received by the receiver equipment, Fig. 5. The impulses of current pass through the winding M to the secondary winding 1 11 and from there to the grid of the tube 14-2. From this tube the impulses pass frointhe plate circuit to the winding 1&6 01: the transformer Y to the secondary winding 14'? of this transformer, whereupon such impulses are amplilied by the three tubes 1 13, 1 1 1, and 1&5.

The current after being finally amplified by the tube 145 causes the energization of relay P, 2, associated with the B frequency switch T2. The operation of relay P completes the circuit of the line relay A2. As eight groups of impulses of tone alternator current are received by relay 1 it in turn transmits eight impulses to the line relay A2, thereby causing the switch T2 to step vertically with its wipers opposite the eighth row of bank contacts. Following this group of impulses, the neXt series of six impulses are likewise received at the switch T2, causing it to rotate and s wipers in engagement with the it i set of contacts in the eighth row. It is the private wiper and the bank contact which it engages.

The operation of relay E2 connects ground from the contacts of relay A2. to the winding of the magnet 0 of the frequency change position relay, causing its movable contact to7 engage the contact connected to conductor 2 Relay M will not fully operate at this time, due to its mechanical structure. The operation of relay E2 connects 125-volt current to conductor 7 extending; to the alternator 87, Fig. 3,-so as to place it in operation and connect ringing current to conductors and 6 extending to the contacts of the code ringing relay D2, Fig. 2. Relay D2 responding to the groups of impulses in response to the operation of the line'relay A2 will connect ringing current to line 86. Relay M associatedwith this line. is partly operated. Its associated line relay L will be disconnected so as not to interfere with T- the application of its ringing current.

Approximately three seconds after the transmittal of the last digit, the relay at the distant station corresponding to relay B, Fig. 1, will restore, as previously described. The restoration of this relay will disconnect the alternator or signalling current and close the microphone circuit for conversational purposes. After the disconnection of the alternator current, relay P of switch T2 will restore and open the circuit of relay A2. The restoration of this latter relay will allow relay B2 to restore and complete the circuit of the release magnet of switch T2, causing it to operate and restore the switch" T2 to its normal position. The restoration of the switch T2 will open the circuit of relay M of line No. 86. The restoration of relay M will allow relay L to be reconnected to the line and to be in position to be 030- erated when the called party answers. When this happens, relay L will operate and close the circuit of relay M. A relay L is slow in restoring, both relays are operated during the same time interval. thus allowing; re lay M to become completely operated, disconnect relay L, and connect the line through. to the line relay A1 of switch T1. This latter relay will now supply tallrin current to the called subscriber, and becoming ener gized, will in turn cause relay B1 to operate and connect ground to relay M to maintain it in its energized position. The operation of relay M also connects ground to relay A and it, upon operating, connects 125-volt our rent to the tone alternator, Fig. 3, to place it in operation. Relay also connects ground to the winding of relay H through the contacts of relay D, and the relay H, upon operating, closes the circuit of the mo tor-start relay K and connects 6-volt battery to the filament of tube 109, Fig. 4, through the contact-s of relay G by way of conductor 15.

Following the operation of relay A and beforev the operation of relay K takes place, ground is connected to the windings of relays G and J as follows:

Ground contacts of relay A, contacts of re lay D, contacts of relay K, conductor 21, contacts of the frequency change position relay, conductor 27, windings of relays G and J to battery.

Relay H which is operated at this time connects ground to the windings of relays G and J through contacts of relays G and F to maintain the last two relays energized after the operation of relay K.

The operation of relay G removes 6-volt battery from the filament of tube 109, Fig. 4, and connects it instead to filament of tube 154, Fig. 5.

The operation of relay G also connect-s ground to the winding of relay S, Fig. 3, by way of conductor 12.

The operation of this latter relay substitutes the B frequency variometer 85 for the A frequency variometer 84 and also substitutes the adjustable set of condensers 82 for the bank 81.

Relay A1 also connects ground to the wlnding of the relay C, Fig. 1, which upon operating removes the negative bias battery from the grid of tube 110, Fig. 4. The relay B becomes energized from the plate circuit of this tube and disconnects the tone alternator signalling current and closes the microphone circuit through the windings 64 and 67 of the transformers, Fig. 8.

At this stage, we have the switch at the distant station corresponding to switch T1, Fig. 1, in its operated position and have the tone alternator current disconnected and the microphone circuit closed.

At the incoming end, we have switch T2, Fig. 2, restored, and in Fig. 1 relays M, A1, B1, A, B, G, H, G, J, and K operated and the switch T1 in its norinal position.

The transmission circuit of the calling sub scriber at the distant station is over the carrier current by way of conductors 40 and 41, through the windings 101 and 140 of Figs. 4 and 5. The current passing through winding 140 is induced in the secondary winding 41. of the transformer, and this current is received at the grid of the detector tube 142 and passes from there to the output or plate circui through winding 201 of the transformer Y, Fig. 5.

Atthis time, winding 146 of the trans- -going A frequency call.

former Y through which the signalling or alternator tone current passes is short-circuited through the contacts of relay J by way of conductors 24 and 25. This current passing through winding 201 induces current in the winding 148 which is in series with winding 113 of the transformer V, Fig. 4. At this time, the winding 200 of the transformer V, Fig. 4, is short-circuited through the contacts of relay J, Fig. 1, by way of conductors l8 and 19.

The current, therefore, passing through winding 201 of the transformer Y, Fig. 5, will have no efi'ect upon the transformer V of Fig. 4, but the current will have effect upon the transformer winding of the coil T, Fig. 4. As the current passes through the primary winding 121 of this transformer, it induces current in the windings which are connected to conductors 1 and 2 which extend to called line 86 through the contacts of relay M associated with that line.

Outgoing call using B frequency In this instance it is necessary to employ the variometer 85, Fig. 3, as for incoming calls using B frequency current. lVith this exception the operation of the apparatus is identical with that in extending an outgoing call using A frequency current.

The party No. 86 originating a call will operate the relays as described for an out- Instead of 4455 2O, he will dial 4555 2O.

The first four impulses will cause the switch T1, Fig. 1, to step vertically and bring the wipers in line with the fourth row of bank contacts. The next series of five impulses will cause the switch to rotate and bring its wipers to rest on the fifth set of contacts.

In this instance wiper P1 will engage contact 45 to which the windings of relays G and J are connected by conductor 27.

Relay E1 becoming energized places ground on contact 45, thus operating relays G and J.

The operation of relay G connects ground to the winding of relay S, Fig. 3, by way of conductor 12, causing it to operate and connect the variometer 85, as previously described.

The remaining groups of impulses perform the same results as before described to select and ring the called party.

At the termination of the conversation the apparatus is restored to its normal condition.

What is claimed is:

1. In combination, a power line, means for superimposing a carrier current thereon to form a channel forsignals, an office including a radio receiver, an automatic switch and a substation provided with a telephone, means controlled by said receiver for operating said switch to select and signal said substation, means for releasing said switch after a predetermined period, and means responsive to the removal of the receiver at the substation for completing a conversational path between said substation and'said carrier cur rent channel provided said switch has previously released.

- 2. In combination, a power line, means for superimposing a carrier current thereon to form a channel forv signals, an ofiice including a radio receiver, an automatic switch and a substation providedwith a telephone, means controlled by said receiver for operating said switch to select and signal said substation, means for releasing said switch after a predetermined period, and means responsive to the release ofsaid switch for completing a conversational path between said substation and said carrier current channel provided the receiver atsaid substation has previously been removed.

3. In a carrier current telephone system, a radio receiver responsive to carrier current of one frequency, a second receiver responsive to carrier current of a different frequency, a radio transmitter normally tuned to the first of said frequencies, an automatic switch, means responsive to the operation of said switch to a predetermined position for preparing a conversational path through said first receiver, and means responsive to the operationof said switch to a second predetermined position fortuning saidtransmitter to the second of said frequencies and for preparing a conversational path through said second receiver. a f 1 4. In a telephone system, a station, a carrier current channel superimposedon a line extending to a distant exchange, means controlled by said station for transmitting selectingimpulses and for talking over the channelby modulating the channel frequency in accordance with dialling and speed currents,

respectively, said speech modulating means being inoperative while dialling is in progress, and means for automatically rendering the speech modulating means operative a predetermined time after said selecting impulses have been transmitted. r

5. In combination, a line, a carrier current channel superimposed on said line, means for modulating the carrier current with current of a lower frequency, means at a calling stationfor transmitting digit impulses by interrupting said modulating current,'a switch responsive to said impulses to select a called station, means for modulating the carrier current by speech currents produced at the calling station, and a timing device for rendering said last means operative a predetermined time after the said impulses have been transmitted.

6. In a telephone system, an exchange, a carrier current channel connecting said exchange with another exchange, a switch in the first exchange operated by a calling party to. select said channel, a source of dialling current associated with the selected channel for modulating the same responsiveto the operation of said switch, and means for operat ing said switch as a repeater to interrupt the circuit of said source and thereby transmit impulses over said channel.

7. In a telephone system, a station, a carrier current channel extending to a distant exchange, an automatic switch controlled by said station, means including said switch for transmitting impulses over saidchannel to select a called line in said exchangeand' for transmitting additional impulses over said channelto signal the selected called station, and a conversational path extending from, said station over said channel to said called line independent of said automatic switch.

8. In atelephone system, a station, a carrier current channel extending to a distant exchange, an automatic switch controlled by said station, means including said switch for transmitting impulses over said channel to select a called line in said exchange, a conversational path extending from said station over said channel to said called line independent of said automatic switch, and means for completing said path a predetermined time after said impulses have been transmitted.

.9. In combination, a line, a carrier current channel superimposed on said line, an automatic switch, a plurality of stations, means for receiving impulses over said channel for. operatingsaid switch to select one of said stations, means in said switch responsive to further impulses received over said channel for signalling the selected station, and a con-.

versational path extendingfrom the selected station over said channel independentof said automatic switch. 7

10. In combination, a telephone exchange, a line extending to another exchange, a trans.- mitter for superimposing a carriercurrent channel on said line, a plurality of local stat tions, an automatic switch common to said stations, means responsive to the operation of said switch by any one of said stations on making a call for preparing said transmitter for operation, and means effective in case the call is for one of said local stations for disabling said first means.

11. In a telephone system, two exchanges, a line connecting said exchanges having a carrier current channel superimposed thereon, a transmitter at one exchange for trans-. mitting carrier current-and for modulating the same in accordance with a low frequency dialling current and with voice frequency currents, a receiver at said exchange tuned to, the same frequency as said transmitter, a similar receiver at the second exchange, a transmitter at said second exchange tuned to the same frequency as said first transmit- 12. In a carrier current telephone system,

an exchange including a plurality of subscribers lines, a receiver at said exchange tuned to the frequency of the carrier current, an automatic switch associated with said receiver, means responsive to the receipt of impulses by said receiver for operating said switch to connect with a called one of said subscribers lines and to signal said line, and means operative a predetermined time after saidgline has been signalled for releasing said switch and for completing a conversational path to said line independent of said switch.

13. In a carrier current telephone system, an exchange, subscribers lines, a receiver at said exchange tuned to the frequency of the carrier current, an automatic switch associated with said receiver, means responsive to the receipt of impulses by said receiver for operating said switch to select a called one of said subscribers lines, means responsive to the receipt of further impulses for caus ing said switch to signal said line in accordance with a predetermined code, andmeans operative a predetermined time after said line has been signalled for releasing said switch.

14. In a telephone system, an exchange, a carrier current channel extending to a second exchange, an automatic switch in the first exchange operated from acalling station to select said channel and to transmit impulses thereover to select a called station in said second exchange, means controlled by said switch for completing a talking circuit between said calling and called stations independent of said switch, and means including said switch for completing local connections within said first exchange, the talking circuits for said local connections extending through said switch.

15. In a carrier current telephone system, an exchange having a plurality of stations, an automatic switch common to said stations and having access thereto, a carrier current 16. In a telephone system, a station, a carrier current channel superimposed on a line extending to a distant exchange, means controlled by said station for transmitting selecting impulses and for talking over .said channel by modulating the channel frequency in accordance with dialling and speech currents, respectively, and means for rendering said dial modulating means inoperative a predetermined time after said selecting impulses have been transmitted- 17. In a telephone system, a line extending to a distant exchange, a station having access to said line, means for superimposing a carrier current channel on said line, means for modulating said carrier current with a dialling current having a particular frequency within the voice frequency range and with speech currents, means controlled. from said station for interrupting said dial-- ling current to transmit digit impulses over said line, and means for preventing said dialling current from interfering with subsequent conversation over said carrier channel by speech current modulation.

18. In a telephone system, a station, a carrier current channel superimposed on a line extending to a distant exchange, means for modulating said carrier current in accordance with a voice frequency alternating-current, means controlled from said station for interrupting said modulating current to transmit dialing impulses over said line, means for automatically cutting off said modulating current after the digit impulses havebeen transmitted, and means for thereafter modulating said carrier current in accordance with voice currents.

19. In a telephone system, a line, means for superimposing a carrier current channel thereon, a station having access to said line, means controlled by said station for modulating said carrier in accordancewith dialling and speech currents, means for maintaining said'sp'eech modulating means inoperative while dialling is in progress, and means for rendering said speech modulating means operative and said dial modulating means inoperativea predetermined time after the carrier current modulated by dialling current has been transmitted over said line.

In witness whereof, I hereunto-subscribe my namethis 28th day of March, A. D. 1928 MARTIN L. NELSON.

In witness whereof, I hereuntosubscribe my name this 28th day ofMarch. A. D. 1928 HARRY E. HERSHEY. 

